1. Field of the Invention
The present invention relates to the growth of nanotubes (NTs). The growth is accomplished by transporting the feedatoms of the NT to the catpar of the NT without the atom being chemically bound to a molecule in the atmosphere environment that surrounds the growing nanotube. The current situation can be illustrated by considering the example of CNTs.
Manmade CNTs are created by various means. Consider one of the most useful techniques, chemical vapor deposition (CVD). Basically, the CVD process involves a carbon bearing gas as a constituent of the atmosphere in a reaction chamber. Some of these gas molecules react with a catpar in the chamber and if the temperature, partial gas pressure and many other parameters are correct, a carbon atom from a gas molecule migrates into or onto the surface of the catpar and a CNT will grow out of the catpar. This process is quite popular because the CVD process, in general, has proven to be extremely useful, over many decades, in other endeavors including semiconductor microcircuit fabrication. However, there are drawbacks when this technology is used for CNT growth.
The first drawback is that although initial growth of the CNTs is quite rapid, the growth quickly slows to a crawl and for all intents and purposes stops. Breakthroughs have been made that allow the growth to continue perceptibly, albeit slowly, but a second problem comes into play. The already formed CNTs are immersed in an environment of hot, carbon bearing gasses. Reactions continue on the surface of the CNTs that create imperfections in their highly structured carbon lattice. These imperfections dramatically degrade the physical properties of the CNTs. The longer the growth continues in this environment, the more damage is done to the CNTs. Therefore significant quantities of long (≧1 centimeter for CNTs, many centimeters for BNNTs), highq CNTs are impossible to fabricate. For over a decade, researchers have been trying to find the “right set” of CVD parameters to produce long, highq CNTs without success.
Causes of the dramatic slowdown of CNT growth during the CVD process are currently understood to include:                1) The accumulation of material on the surface of the catpar, suspected to be amorphous carbon. This coating reduces the surface area of the catpar thereby decreasing the opportunity for carbon atoms, appropriate to combine with the growing CNT, to either pass into the catpar or migrate on its surface to the CNT growth location. Thus CNT growth is slowed or terminated.        2) The effect of Ostwald ripening tends to reduce the size of small catpars and increase the size of large catpars by mass transfer from the small to the large. Conceptually this is because small particles are thermodynamically less stable than larger particles. This thermodynamically-driven process is seeking to minimize the system surface energy. The catpar size is important since CNT growth will cease (or not begin in the first place) if the catpar is too large or too small.        3) Although substrates upon which CNTs are grown can be many different substances, the most common substrate is silicon dioxide, in part because of the decades of experience with it in the semiconductor industry. Silicon dioxide was thought to be impervious to catalyst elements, but in CNT fabrication it has been found that at least some catalyst materials can diffuse into the silicon dioxide layer. Thus the effective size of the catpar gets smaller and can become incapable of supporting CNT growth. Other substrates may be porous to catalyst materials as well.        
2. Description of the Prior Art
U.S. Pat. No. 7,045,108 describes the growth of CNTs on a substrate and the subsequent drawing of those CNTs off the substrate in a continuous bundle. The abstract states: A method of fabricating a long carbon nanotube yarn includes the following steps: (1) providing a flat and smooth substrate; (2) depositing a catalyst on the substrate; (3) positioning the substrate with the catalyst in a furnace; (4) heating the furnace to a predetermined temperature; (5) supplying a mixture of carbon containing gas and protecting gas into the furnace; (6) controlling a difference between the local temperature of the catalyst and the furnace temperature to be at least 50 .degree. C.; (7) controlling the partial pressure of the carbon containing gas to be less than 0.2; (8) growing a number of carbon nanotubes on the substrate such that a carbon nanotube array is formed on the substrate; and (9) drawing out a bundle of carbon nanotubes from the carbon nanotube array such that a carbon nanotube yarn is formed.
The technique described in the previous paragraph is a representative example of the popular and useful “forest growth” of CNTs and the drawing of a CNT bundle from the forest. It does not discuss any technique for mitigating the causes for CNT growth slowdown.
U.S. Pat. No. 8,206,674 describes a growth technique for boron nitride nanotubes (BNNTs). From the abstract: Boron nitride nanotubes are prepared by a process which includes: (a) creating a source of boron vapor; (b) mixing the boron vapor with nitrogen gas so that a mixture of boron vapor and nitrogen gas is present at a nucleation site, which is a surface, the nitrogen gas being provided at a pressure elevated above atmospheric, e.g., from greater than about 2 atmospheres up to about 250 atmospheres; and (c) harvesting boron nitride nanotubes, which are formed at the nucleation site.
The above technique forms centimeter long BNNT using laser ablation of the boron into a nitrogen atmosphere. The growth occurs at a rough spot around the ablation crater and the growth streams in the direction of the nitrogen flow. A catalyst material need not be present. The technology does not allow for the control of growth or the use of this laser ablation technology to grow CNTs.
U.S. Pat. No. 8,173,211 describes CVD CNT growth process that is continuous. From the abstract: A method of production of carbon nanoparticles comprises the steps of: providing on substrate particles a transition metal compound which is decomposable to yield the transition metal under conditions permitting carbon nanoparticle formation, contacting a gaseous carbon source with the substrate particles, before, during or after said contacting step, decomposing the transition metal compound to yield the transition metal on the substrate particles, forming carbon nanoparticles by decomposition of the carbon source catalyzed by the transition metal, and collecting the carbon nanoparticles formed.
The technique described in the previous paragraph is the technique in which the catalyst is dispersed into the carbon-bearing gas flow of the reactor. It produces CNTs of up to approximately 0.5 mm in length. The CNTs appear as smoke and can be drawn off continuously. However, the technology has been unable to grow long, highq CNTs.